How real cultures name their rivers
Hydronyms (river names) tend to be the oldest toponyms in any region. Long before cities existed, rivers had names. Danube has Indo-European root from 5000 years ago; Seine comes from Gaulish; Paraná means 'kin of the sea' in Guaraní. In your fantasy worldbuilding, river names should sound more archaic than city names.
The most common real patterns are: physical description (Black River, Red River, Salt River), associated fauna (Wolf River, Salmon River), pre-Roman or indigenous root (Seine, Tagus, Pilcomayo), and historical event (Río de la Plata for silver trade, Bloody River for battle). Your map gains realism by combining these four types.
Some rivers carry the name of the first important city they cross; others, the reverse, lend their name to the city: Buenos Aires sits on the Río de la Plata, but Mendoza gives name to the river crossing it. This name-city/river duality is rich worldbuilding for your novels.
Why rivers matter in worldbuilding
Rivers aren't decoration: they're economic and political arteries. Every ancient civilization rose near a major river: Egypt-Nile, Mesopotamia-Tigris/Euphrates, Indus, Yangtze, Amazon. In your fantasy map, important cities should be on navigable rivers or strategic confluences.
Rivers are also natural political borders. The Rhine separates Germanic from Latin culture; the Danube divided the Roman Empire; the Rio Grande divides Mexico from US. In your world, opposing kingdoms can have a river as border, with all narrative consequences: smuggling, espionage, ford negotiations, battles at bridges.
Sacred rivers add religious layer. The Ganges, Jordan, Euphrates: all have spiritual significance in their respective cultures. If your world has such a river, define: what rituals are practiced? who can drink it? what pilgrimages does it receive? Those details give texture.
For RPG campaigns, rivers are obvious travel routes: boat descent, aquatic dangers (creatures, rapids, shipwreck), encounters with fishermen and smugglers. Take advantage of fluvial geography as narrative device, not just map drawing.
Common mistakes in fictional river names
Names too modern: Express River or Trans-Rapid Swift River don't work. Real hydronyms are almost always archaic. If you need to give a new name to a river in your world, justify: some catastrophe recently renamed it, or it's an artificial river created by magic/technology.
Repeated generic names: if your map has Black River, Long River, Cold River, you lose differentiation. Mix physical names with legendary names: Black River + Flow of the Pact + Vein of the Mother on the same map enriches texture.
Forgetting tributaries: great rivers rarely are just one. The Amazon has hundreds of tributaries with own names. Your map can show two or three main tributaries with names distinct from the main river. The Flow of the Pact receives water from the Thread of the Swan and Branch of the Wolf.
Lack of consistent geography: if all your rivers go from north to south on your map, it seems artificial. Real rivers are born in mountains and flow toward seas or oceans: draw geography first (ranges and coasts), then trace rivers coherently. Hydrological consistency is what separates professional worldbuilding from improvised map.
Details that bring a fictional river alive
After the name, define five traits. Water color: is it murky, crystal, dark, milky? The Amazon and the Brazilian Black River are famous for their 'meeting of waters' where two colors don't mix for kilometers. Any such striking trait passes into folklore.
Characteristic fauna: what fish and creatures live in it? In fantasy, you can have mythic species: aquatic serpents, ondines, singing fish. Fauna defines local gastronomy, occupations (specific fishermen), dangers (crocodiles, electric eels, something darker).
Seasonal rhythm: does the river swell with spring melt? dry up in summer? have catastrophic floods? These cycles define riverside culture: when they sow, when they sail, when they evacuate. Nile floods created ancient Egypt; Mekong cycles sustain entire cultures.
Notable confluences: where two rivers meet tends to be important city or sacred site. Confluence of the Three Flows is a name suggesting precise geography and associated city.
Associated legends: each significant river has 1-3 legends locals tell. A god who drowned there, a lover who jumped from a bridge, an inexplicable wreck. These legends are part of the cultural landscape and enrich any scene happening near the river.